Coriolis mass flow and density meters are considered the flow metering solution of choice for many precision flow applications. Since their introduction to the mainstream flow metering community in the early 1980's, coriolis meters have grown into one of largest and fastest growing market segments, representing roughly $400 million annual sales on approximately 100,000 units. The success of coriolis meters has been attributed to many factors, including its accuracy, reliability, and ability to measure multiple process parameters, including mass flow and process fluid density. However, despite this long list of attributes, coriolis meters have significant limitations regarding aerated liquids. Although the mass flow rate and process fluid density measurements determined by coriolis meters are derived from independent physical principles, the accuracy of both are significantly degraded with the introduction of small amounts of entrained gases. This paper develops and lumped parameter aeroelastic model for coriolis meters operating on aerated fluids. The model is to examine the influence of several key, non-dimensional parameters that influence the effect on aeration on the mass flow and density reported by commercially available coriolis flow meters.
Coriolis meters are widely used for industrial flow measurement, representing one of the largest and fasting growing segments in the industrial flow meter market. Coriolis meters have the reputation for high accuracy and provide mass flow and density as their basic measurements.
Since the technology was first adopted by industry beginning in the 1980's, Coriolis meters have developed the reputation as a high priced, high accuracy meter for use in high value applications—predominately within the chemical processing industry. However, despite their success, Coriolis meters have been plagued by poor performance in two-phase flows, predominately bubbly flows of gas/liquid mixtures.
Coriolis meters have two fundamental issues with aerated or bubbly flows. Firstly, bubbly flows present an operability challenge to coriolis meters. Most coriolis meters use electromagnetic drive actuators to vibrate the flow tube at it natural frequency. The meters rely on the vibrating tubes to generate the corilois forces which causes one leg of the flow tube to lag the other. The corilois forces, and hence phase lag, are ideally proportional to the mass flow through the flow tube. The tubes are typically excited at, or near a resonant frequency, and as such, the excitation forces required to maintain a specified vibration amplitude in the tubes is a strong function of the damping in the system. Single phase mixtures introduce little damping to the vibration of the bent tubes, however, the amount of damping in the system dramatically increases with the introduction of gas bubbles. As a result, more power is required to maintain vibration in the tubes in bubbly flows. Often more power is required than is available, resulting in the “stalling” of the Corilois meter.
Furthermore, coriolis meters often require significant time to adjust for the often rapid changes in flow tube resonant frequencies associated with the onset of bubbly or aerated flows. These time-delays, for which the flow tube is essentially stalled, greatly diminish the utility of coriolis meter in many applications where two phase flow and transient response are important such as batch processed. This stalling problem has been and is currently being address by many manufactures.
Secondly, multiphase flows present an accuracy challenge. The accuracy challenge presented by aerated flow regimes is that many of the fundamental assumptions associated with the principle of operation of Corilois meters become increasingly less accurate with the introduction of aerated flow. The present invention provides a means for improving the accuracy of Coriolis meters operating on all types of fluids, with particular emphasis on enhancing the accuracy for operating on two phase, bubbly flows and mixtures.